The present invention is directed to the use of ribbons and tapes as a source of ink or other fluid for typing and printing, wherein images are formed (and removed ) by impressing a type on a sheet of paper through the medium of the ribbon or tape. More particularly, the invention pertains to a cartridge for use in an impact printing device that stores a quantity of fluid impregnated ribbon or tape, and in addition, feeds the ribbon or tape through the printing device as new characters are typed so that a fresh supply of the fluid is maintained. Still more particularly, the invention relates to an improved cartridge drive system for feeding a ribbon or tape through a cartridge during a printing operation.
It is common in impact printing to use a ribbon or tape impregnated with ink, correction fluid or the like to form and remove images on a sheet of paper by striking or impacting the paper with a type character through the medium of the ribbon or tape. In the impact printing art, the term "ribbon" is commonly used to describe an ink-bearing medium whereas the term "tape" is often used to denote media containing a correction fluid for removing or covering a previously typed character. For convenience, the term "ribbon" will be used throughout this document as including both ribbon and tape as defined above.
Most ribbon cartridges for impact printing use a gear drive system to continuously feed ribbon through the printing device in which the cartridge is installed. A drive gear in the cartridge is positioned to engage a drive mechanism on the printer, which rotates continuously, or as each character is printed, depending on printer design. This rotation causes the drive gear and an associated intermeshing idler gear to rotate, which pulls the ribbon through the cartridge. The ribbon is usually formed as a continuous loop, the bulk of which is stored in a fan-folded or random arrangement known as a "fabric pack" in a ribbon chamber within the cartridge. From the ribbon chamber, the ribbon feeds through a ribbon outlet, past a print head, and through a ribbon inlet to return to the ribbon chamber.
In order to feed ribbon through the cartridge, the gear drive system must pull the ribbon from the ribbon outlet, draw it into the ribbon inlet, and discharge it into the ribbon chamber. This operation should require only minimal mechanical torque in order to reduce ribbon web tension and avoid imposing excessive power or mechanical demands on the printing device. Moreover, the drive gear system must be capable of accommodating momentary increases in ribbon tension caused when the ribbon fabric becomes partially trapped within the ribbon chamber. This often results from a "buried loop", which is caused when one partial loop of the ribbon fabric is trapped between the side wall of the cartridge and the fabric pack or within the pack itself. Excessive ribbon tension resulting from a buried loop can cause the ribbon to slip between the gears. Such "stalling" can cause either light and dark print or even cartridge or printer failure.
Prior art ribbon cartridges commonly use either a fixed gear drive system or a spring loaded gear system. In a fixed gear system a pair of spur gears frequently having deep, large teeth are used to pull the ribbon. One of the gears is the drive gear and the other gear is an intermeshing idler gear. Both are mounted on fixed rotational axes, typically holes in the top and bottom of the cartridge. A disadvantage of fixed gear systems is that the spacing between the gear axes cannot be precisely controlled due to manufacturing tolerances inherent in molded plastic components. This can cause wide swings in performance between the extremes of binding with high torque (causing possible machine damage or stalling) and fabric slippage resulting in light/dark print.
In a spring loaded gear system the drive gear is rotatably mounted in a fixed axis while an intermeshing idler gear is slideably mounted to allow lateral displacement of the idler gear rotational axis toward the drive gear axis. Uniform drive torque is maintained by resiliently biasing the idler gear toward the drive gear using a spring loading system. A disadvantage of the spring loaded gear system is that more parts are required, which increases the cost of labor and materials, and adds more tolerances for potential variances. In addition, a higher drive torque is generated by the spring force required to maintain desired traction on the ribbon fabric. Spring loaded gear systems also allow backward rotation of the gears unless an additional ratchet is utilized to allow one-way rotation only. Reverse manual rotation frequently results in jams or failure of the system.
In view of the foregoing, there is an evident and presently unfulfilled need in the art for a ribbon cartridge and ribbon drive system that overcomes the problems and disadvantages of previous designs, including, in particular, the problems of excessive drive torque and undesirable drive slippage under high load conditions.